1. Field of the Invention
This invention relates to an apparatus for heat sealing thermally fusable tabs provided along the edges of a jacket for accommodating a flexible magnetic disc.
2. Description of the Prior Art
A magnetic disc jacket of the type described is composed of a synthetic resin sheet such as a polyvinyl chloride sheet having a non-woven liner bonded to the inner surface thereof. The jacket, shown in an unfolded condition in FIG. 1, comprises a planar, generally rectangular sheet 1 provided beforehand with openings constituting circular center holes 2a, 2b, elongate head windows 3a, 3b and index holes 4a, 4b on left and right halves of the sheet, respectively, divided by an imaginery center line L. The sheet 1 also has thermally fusible tabs 5a, 5b, 5c for heat sealing provided on the upper, lower and left edges, respectively, of the left half of the sheet as viewed in FIG. 1. The sheet 1 is folded upon itself along the center line L to bring the center holes 2a, 2b, head windows 3a, 3b and index holes 4a, 4b into register. The opposing heat sealing tabs 5a, 5b are then folded onto the right half of the sheet to provide a generally square-shaped jacket closed along three sides, namely the sides corresponding to tabs 5a, 5b and center line L, as illustrated in FIG. 2. Next, the folded tabs 5a, 5b are partially heat sealed by thermal pressing using a sealing mold, and a flexible magnetic disk is inserted into the jacket from its open edge along the as yet unfolded tab 5c. This is followed by folding the tab 5c onto the right half of the sheet and applying heat and pressure to seal the tab 5c by heat sealing.
In effecting the partial heat sealing of the folded tabs 5a, 5b of the jacket shown in FIG. 2 by heating and pressing, the base on which sheet 1 is placed for being pressed is, according to the prior art, held at a temperature at or slightly below room temperature so that portions other than the base will not be influenced by the heat involved in the heating and pressing step. As a result, when the folded jacket is placed on the base and is then heated by the sealing mold, there is a difference is temperature between the side of the jacket facing the mold and the side facing the base. Consequently, when the jacket cools following the heat sealing of the tabs, distortion sets in owing to residual thermal stress so that the jacket tends to warp as shown at numeral 50 in FIG. 3. Owing to this reduced flatness of the jacket, an appropriate torque cannot be obtained when rotating a magnetic disc inserted into the jacket. In addition, a warped jacket loaded into a magnetic disc drive is likely to damage the drive.